I. Field of the Invention.
The present invention is directed toward an underwater trenching system. More particularly, the present invention digs an underwater trench for burying a cable or pipeline and backfills the trench using the same material that was removed to create the trench.
II. Description of Related Art.
It is frequently necessary or desirable to bury a pipe or cable at the bottom of a waterway, such as a river, lake or sea. Buried lines often include, for example, natural gas lines, petroleum products lines, fiber optic cables, telephone cables, and so forth.
Many such cables or pipelines are simply laid along the bottom of a waterway and left exposed, to be buried by the action of the currents. In other uses, a trenching tool, such as a water jet, a cutter head, or a scoop, or clam shell digger digs a trench around the pipe, which settles into the trench. The bottom matter, sediment, or spoils, blasted away by the cutting head, regardless of the type, is dispersed in the surrounding water where it remains in suspension for a considerable time before eventually settling to the bottom. The sediment may form a large cloud within the body of water, or be carried downstream many miles before eventually settling out of suspension. It has become known that the debris from such trenching operations can cause serious environmental damage to plants, animals, fishes, the water, microorganisms and so forth, particularly in fragile ecosystems.
Typically, no effort is made to cover the pipe or to fill the trench. Only the action of underwater currents may, or may not, fill the trench. In some applications, particularly those in which the trench is dug by a clam shell digger, the spoils are left alongside the trench, and later may be moved to bury the pipeline. When this technique is used, the waterway is disturbed by suspended sediment twice and to the same degree. In addition, filling the trench this way requires two passes along the entire route of the underwater pipeline, doubling the cost of labor and the commitment of capital resources.
In short, all currently known equipment and methods for underwater trenching create large clouds of silt and debris that remain in suspension for a long time and seriously disrupt the ecology of the waterway. And backfilling the trench doubles the disruption.
A typical prior art underwater excavator or trencher is shown in Notarbartolo et al U.S. Pat. No. 2,659,211, which employs an excavator having a water jet which includes an auxiliary stream for unburying a cable. The spoils are merely blasted away wherever the water jet currents take them.
Another submarine pipeline trencher is disclosed in Roy U.S. Pat. No. 3,722,224, which teaches the use of a water jet and a vacuum unit that prevents spoils from resettling into the excavated trench, but the vacuumed material is simply discharged laterally and rearwardly of the apparatus, the object being merely to keep the spoils out of the newly formed trench. A pipe-burying jet sled is disclosed in Gaspar U.S. Pat. No. 4,295,757, which employs jetting legs for the water jets and, in addition, pressurized air for enhancing removal of sediment, which is discharged directly from the apparatus into the water, thereby forming large clouds of polluted water.
Another water jet-type apparatus is disclosed in Berti et al U.S. Pat. No. 4,479,741 which sucks the spoils out of the trench through scooping units by vacuum, allowing the matter to move freely in the water away from the trench.
While these references show that it is well known to use a tool such as a water jet for creating an underwater trench, the prior art does not show any means for containing or minimizing the spread of the spoils during trenching, or for disposal of the spoils, or for re-filling the trench. It has become apparent that large clouds of spoils suspended in the waterway are a hazard to all marine life, seriously disrupts the ecology of the waterway, despoil the appearance of the waterway, and should be avoided if possible.
Accordingly, there is a need for an underwater trenching system that contains much or all of the spoils during trenching to prevent their being dispersed in the waterway; that backfills the trench after the pipe has settled into the trench, using the same spoils removed to make the trench; and simplifies the trenching and backfilling process, thereby reducing the time and cost of laying underwater pipelines and cables.